The present invention pertains to microlithography systems that employ a charged particle beam to transfer a pattern (such as a circuit pattern for an integrated circuit) onto a sensitive substrate (such as a semiconductor wafer). More specifically, the invention pertains to methods and apparatus for reducing aberrations in such systems.
There has been much recent progress in semiconductor processing technology, including so-called xe2x80x9cmicrolithographyxe2x80x9d technology. Current optical microlithography apparatus utilize light (typically ultraviolet light) for exposure. To further improve the resolution of microlithography apparatus, light having increasingly shorter wavelengths is preferably used. However, there are practical limits to the extent to which the wavelength of light can be reduced for microlithography.
To solve such problems, X-ray microlithography has been under development. However, X-ray technology for microlithography is plagued with many serious problems, including a current inability to fabricate an acceptable reticle for X-ray lithography. Consequently, practical X-ray microlithography has not yet been realized.
Another approach to improving the resolution of microlithography has been the use of charged-particle-beam (CPB) apparatus, such as electron-beam apparatus. Fortunately, many aspects of CPB optics are well understood in view of many years"" experience in using CPB optics in instruments such as electron microscopes and the like. With respect to CPB microlithography, various optical configurations have been proposed, such as MOL (Moving Objective Lens; Goto et al., Optik 48:255, 1977), VAL (Variable Axis Lens; Pfeiffer and Langner, J. Vac. Sci. Technol. 19:1058, 1981), and VAIL (Variable Axis Immersion Lens; Sturans et al., J. Vac. Sci. Technol. B8:1682, 1982). 
However, whenever a circuit pattern is exposed using a CPB optical system such as any of the listed systems, image blur generated by geometric aberrations of the CPB optical system is very high, which is problematic for practical applications. In addition, in CPB exposure apparatus based on a multiple-deflection theory (directed to reducing third-order aberrations, Hosokawa, Optik 56:21, 1980), a number of deflectors equal to the number of aberrations to be eliminated is used. The principal aberrations discussed in the Hosokawa paper are longitudinal coma aberration, radial coma aberration, astigmatism, and chromatic aberrations; hence, at least four deflectors are required to reduce these aberrations. An additional deflector is normally used for controlling the angle of incidence of the beam onto the reticle and substrate. Hence, five deflectors are typically required. In the Hosokawa paper, a respective linear equation is used to define the current applied to each deflector, and the linear equations are combined to form a set of simultaneous linear equations. The set of simultaneous linear equations is then solved to provide a respective current for each deflector. Unfortunately, however, in such a scheme the trajectory of the beam is unnatural. (xe2x80x9cUnnaturalxe2x80x9d in this context means that the trajectory Wm of the beam changes greatly. The CPB optical system has three principal rays: Wa, Wb, and Wm. Chu et al., Optik 61:121, 1982. Wa and Wb are determined by lens conditions, and Wm is determined by both the lenses and the deflectors.) As a result, higher-order aberrations (i.e., higher order than third order) increase sharply even when third-order aberrations are largely eliminated. Moreover, the solutions to the set of simultaneous linear equations change continuously and thus require constant recalculation.
Furthermore, in conventional systems as summarized above, aberrations are calculated only up to the third-order aberrations; no calculations are performed of fifth-order or higher-order aberrations. However, in order to achieve the requisite imaging accuracy using a CPB microlithography apparatus to expose a pattern having a critical feature dimension of less than 0.15 xcexcm, fifth-order and higher-order aberrations must be reduced as much as possible.
By way of example, in conventional SMD optical systems, whenever the magnification (M) exhibited by such a system is equal to 4 (i.e., whenever M=4) and the inside radius of a deflector on the image side is 25 mm, the inside radius of a deflector on the object side is 100 mm. In conventional SMD systems, deflectors are usually situated along the optical axis inside one or both lenses. The example situation just mentioned would require that the inside radius of the lens on the object side be very large. As a result, not only must the size and the weight of the lens itself be very large, but also the radius of a xe2x80x9clens columnxe2x80x9d (i.e., lens housing) containing the lens must be even larger. This results in a corresponding increase in the size and complexity of the microlithography apparatus utilizing such a lens. Other detriments with such size increases are greater deviations from design specification due to assembly errors and the like (e.g., positional errors of the optical axis and displacements of the respective rotation angle and the like of the deflectors).
Also, with conventional SMD optical systems, the number of deflectors for correcting aberrations is limited, which correspondingly limits the degrees of freedom for aberration-reducing adjustments. Consequently, blur due to aberrations is very sensitive to any displacement of the axial position of the deflectors and any displacement of the angular orientation of each deflector radially about the optical axis. This situation greatly increases the difficulty of performing aberration-reducing micro-adjustments of the SMD optical system. The end result is that achieving a level of performance from a conventional CPB microlithography apparatus has tended to fall substantially below design specifications.
In view of the foregoing shortcomings of the prior art, an object of the invention is to provide projection-optical systems for CPB microlithography apparatus that exhibit satisfactorily reduced aberrations and blur, even with a large main field.
According to one aspect of the invention, apparatus are provided for projecting an image of a pattern, defined by a reticle, onto a substrate using a charged particle beam. An embodiment of such an apparatus comprises a projection-optical system situated and configured to receive a charged particle beam passing through an illuminated region (i.e., a region irradiated by the charged particle beam) of the reticle and form an image of the illuminated region on a corresponding region of the substrate. The apparatus also comprises six deflectors associated with the projection-optical system. The deflectors are configured and situated to correct an on-axis aberration of the beam and a corresponding off-axis aberration of the beam, wherein the off-axis aberration is corrected substantially equally with the correction of the on-axis aberration. The projection-optical system in such an embodiment preferably satisfies a Symmetric Magnetic Doublet (SMD) condition. Also, the deflectors preferably have axial positions, relative to an axial position of the substrate, of Z1-Z6, respectively, that satisfy the expressions:
Z1=(xe2x88x92L)xe2x88x92M(Z6)
Z2=(xe2x88x92L)xe2x88x92M(Z5)
Z3=(xe2x88x92L)xe2x88x92M(Z4)
wherein an axial direction leading from the substrate to the reticle is regarded as a negative axial direction, xe2x80x9cLxe2x80x9d is the xe2x80x9ccolumn lengthxe2x80x9d of the projection-optical system, and 1/M is the demagnification ratio of the projection-optical system. (An axial direction leading from the substrate to the reticle is regarded as a negative axial direction.)
According to another embodiment, an apparatus is provided that comprises a projection-lens system configured and situated so as to perform the following: (1) receive a charged particle beam passing through an illuminated region of the reticle and form an image of the illuminated region on a corresponding region of the substrate, (2) form a lens field, and (3) satisfy a SMD condition. The apparatus also comprises a deflector system configured and situated so as to form a deflection field. The deflector system comprises a first set of multiple (at least two) deflectors axially arranged on a reticle side of the projection-lens system, and a second set of multiple (at least two) deflectors axially arranged on a substrate side of the projection-lens system. The deflector system performs a correction of third-order aberrations of the beam sufficiently such that off-axis third-order aberrations are corrected substantially equally with correction of on-axis third-order aberrations. The respective inside radii of the deflectors of the first and second sets are configured so as to reduce fifth-order blur generated by deflection of the beam. In addition, the inside radius, outside radius, and axial length of the deflectors in the first set are preferably M times the inside radius, outside radius, and axial length, respectively, of the deflectors in the second set, wherein 1/M is the demagnification ratio of the projection-lens system.
Each of the first and second sets of deflectors can comprise xe2x80x9cnxe2x80x9d respective deflectors, wherein xe2x80x9cnxe2x80x9d is an integer greater than two. With such a configuration, the respective nth deflectors of the first and second sets are preferably axially situated closest to each other of all the deflectors in the first and second sets, respectively. Furthermore, the respective nth deflector of the first and second sets preferably have a smaller inside radius, outside radius, and axial length of the other deflectors in the respective first and second sets.
According to another aspect of the invention, certain improvements are provided to apparatus for projecting an image of a pattern, defined by a reticle, onto a substrate using a charged particle beam. A first set of xe2x80x9cnxe2x80x9d deflectors (Da1, Da2, . . . , Dan) is provided (wherein xe2x80x9cnxe2x80x9d is at least two) on the reticle side of the projection-optical system of such an apparatus. A second set of xe2x80x9cnxe2x80x9d deflectors (Db1, Db2, . . . , Dbn) is provided (wherein xe2x80x9cnxe2x80x9d is at least two) on the substrate side of the projection-optical system. The nth deflector Dan in the first set has an inside radius, outside radius, and axial length that individually are smaller than respective such dimensions of all other deflectors of the first set. Similarly, the nth deflector Dbn in the second set has an inside radius, outside radius, and axial length that individually are smaller than respective such dimensions of all other deflectors of the second set.
According to yet another aspect of the invention, a projection-optical system is provided that comprises a projection lens and a deflector system. The projection lens is situated and configured so as to form a charged particle beam passing through an illuminated region of the reticle and form an image of the illuminated region on a corresponding region of the substrate. The deflector system includes a first deflector set associated with an xe2x80x9cimage sidexe2x80x9d of the projection lens (side closer to the image formed by the projection lens), and a second deflector set associated with an xe2x80x9cobject sidexe2x80x9d of the projection lens (side closer to the object). Each of the first and second deflector sets comprises multiple deflectors. At least one deflector of the first set comprises an axial array of micro-deflectors each having a similar radial angular orientation as the respective deflector. Similarly, at least one deflector of the second set comprises an axial array of micro-deflectors each having a similar radial angular orientation as the respective deflector. Each micro-deflector of the first set is preferably independently and adjustably energizable such that the array of micro-deflectors of the first set collectively produces a deflection field similar to a deflection field that otherwise would be produced by the respective deflector in the first set. Similarly, each micro-deflector of the second set is preferably independently and adjustably energizable such that the array of micro-deflectors of the second set collectively produces a deflection field similar to a deflection field that otherwise would be produced by the respective deflector in the second set. The micro-deflectors preferably satisfy the expressions:
0.02xe2x89xa6R1/Kxe2x89xa60.20
0.02 less than R2/Kxe2x89xa60.30
0.01xe2x89xa6L/Kxe2x89xa60.05
0.001xe2x89xa6S/Kxe2x89xa60.05
wherein R1 is the inside radius of the micro-deflectors in a set, R2 is the outside radius of the micro-deflectors in the set, L is the axial length of the micro-deflectors, S is the axial spacing between the micro-deflectors, and K is an axial distance between the reticle and the substrate.
According to yet another aspect of the invention, projection-optical systems for charged-particle-beam microlithography apparatus are provided. According to a representative embodiment, the apparatus comprises a projection lens and a deflector system. The projection lens is situated and configured to receive a charged particle beam passing through an illuminated region of the reticle and form an image of the illuminated region on a corresponding region of the substrate. The deflector system is associated with the projection lens and comprises a first and a second deflector set. The first set comprises multiple micro-deflectors linearly arrayed in an axial direction between the object surface and aperture surface of the projection lens, and the second set comprises multiple micro-deflectors linearly arrayed in an axial direction between the image surface and the aperture surface of the projection lens. The micro-deflectors preferably satisfy the expressions:
0.02xe2x89xa6R1/Kxe2x89xa60.20
0.02 less than R2/Kxe2x89xa60.30
0.01xe2x89xa6L/Kxe2x89xa60.05
0.001xe2x89xa6S/Kxe2x89xa60.05
wherein R1 is the inside radius of the micro-deflectors in a set, R2 is the outside radius of the micro-deflectors in the set, L is the axial length of the micro-deflectors, S is the axial spacing between the micro-deflectors, and K is an axial distance between the reticle and the substrate.
According to yet another aspect of the invention, projection-optical systems are provided for charged-particle-beam microlithography apparatus. A representative embodiment comprises a projection lens and a deflector system. The projection lens is situated and configured as summarized above. The deflector system comprises first and second deflector sets. The first deflector set comprises three deflectors and is situated between the object and aperture surfaces of the projection lens. The second deflector set comprises three deflectors and is situated between the aperture and image surfaces of the projection lens. The first and second sets serve to impart respective corrections of off-axis third-order aberrations and on-axis third-order aberrations, wherein the correction of the off-axis aberrations is substantially equal to the correction of the on-axis aberrations. Each deflector in each of the first and second sets comprises respective multiple micro-deflectors linearly arrayed in an axial direction. Each micro-deflector preferably satisfies the expressions:
0.02xe2x89xa6R1/Kxe2x89xa60.20
0.02 less than R2/Kxe2x89xa60.30
0.01xe2x89xa6L/Kxe2x89xa60.05
0.001xe2x89xa6S/Kxe2x89xa60.05
wherein R1 is the inside radius of the micro-deflectors in the respective set, R2 is the outside radius of the micro-deflectors in the set, L is the axial length of the micro-deflectors, S is the axial spacing between the micro-deflectors, and K is an axial distance between the reticle and the substrate.
By substituting an array of micro-deflectors for an aberration-correcting deflector in a deflector set, the inside radius and outside radius of the deflector system can be made smaller than practical when the deflector system comprises only deflectors. This allows downsizing of the overall microlithographic apparatus.
Also, by forming a deflection field using an array of micro-deflectors rather than a deflector, it is possible to more accurately adjust the electrical current supplied to the deflector group by independently adjusting the respective currents supplied to the individual micro-deflectors. This facilitates easier maintenance and alignment of the microlithographic apparatus
The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.